The present invention relates to the field of injection molding of plastic materials, and in particular to gas assisted molding.
Injection molding of plastic materials is well known and widely practiced as a means of manufacturing an ever-increasing diversity of plastic components for industrial and consumer use. During the last two decades, versions of the process globally referred to as xe2x80x9cgas assisted moldingxe2x80x9d have been developed and used to overcome some of the problems inherent in conventional molding, and to reduce costs and improve the quality of the final products.
In conventional gas molding, a gas, such as nitrogen, is injected into the molten plastic material after it has entered the mold. The low viscosity gas flows into the paths of least resistance within the more viscous plastic, thereby forming hollow channels within the plastic. The process is particularly beneficial for thick section moldings, such as handles, and weight savings of up to 45% or more can result. Also, the molding time cycles can be substantially reduced. In multi-section moldings, the injected gas tends to flow into the thicker sections, again forming hollow continuous channels through which pressure may be transmitted through the medium of the gas. This adds to the scope of the designer and removes some of the design restrictions of conventional molding.
One conventional gas assisted plastic molding process partially fills the mold cavity with an accurately controlled shot volume of plastic. Gas is then injected in order to continue the flow of plastic so that the cavity is filled with plastic and gas. The gas is used to exert outward pressure on the plastic material forcing it against the mold cavity surfaces, thereby achieving a good replication of the mold surface on the plastic molded surface. After the plastic has solidified, the gas pressure is reduced, the gas is exhausted to atmosphere, and the mold is opened and the part ejected. This is sometimes referred to as a xe2x80x9cshort shotxe2x80x9d process.
In another method, the mold cavity is completely or substantially filled with plastic material and then instead of injecting or packing more plastic into the cavity to compensate for the volumetric shrinkage of the plastic as it cools and solidifies, gas is injected into the plastic so that the gas expansion compensates for the plastic contraction. In practice, the initial gas penetration will continue to expand during the cooling cycle while the plastic is shrinking in volume. This is sometimes referred to as a xe2x80x9cfull shotxe2x80x9d process.
In the xe2x80x9cfull shotxe2x80x9d process, it is sometimes difficult to achieve sufficient gas penetration along intended gas channels because there is insufficient volumetric shrinkage of the plastic to provide space for the gas. In such cases, a method of enabling some plastic to outflow from the mold cavity into overflow wells or xe2x80x9csecondaryxe2x80x9d cavities is helpful in providing space for the gas expansion.
In the xe2x80x9cshort shotxe2x80x9d method, some moldings may also be difficult to fill with plastic and gas to the extremities of the molded article cavity. If the shot volume is too little, the gas may break through the leading edge of the plastic material during filling, thereby losing control of the gas. If the shot volume is too high, the gas will not reach the extremities of the cavity. Therefore thick section moldings using the xe2x80x9cshort shotxe2x80x9d process can also benefit from an additional displacement of plastic from the article cavity into an overflow cavity.
A method of at least partially filling the cavity before injection of the gas is described in U.S. Pat. No. 5,098,637. However, in order to use the method of this patent successfully, it is necessary to accurately control the shot volume for both xe2x80x9cshort shotxe2x80x9d and xe2x80x9cfull shotxe2x80x9d methods, because there is no resistance to prevent the plastic from flowing into and filling the overflow cavity before the gas is injected.
In the xe2x80x9cshort shotxe2x80x9d process, the flow of plastic is temporarily stopped at the end of the filling sequence, and then typically there is a delay of up to five seconds before injection of the gas urges the plastic forward to complete the filling of the article cavity with plastic and gas and the overflow cavity with plastic. In the xe2x80x9cfull shotxe2x80x9d process, the mold cavity is filled with, or substantially filled with, molten plastic and the gas is injected to compensate for the volumetric shrinkage of the plastic and to displace plastic into the overflow cavity. In both cases, it is not feasible to apply xe2x80x9cpacking pressurexe2x80x9d by the molding machine because there is nothing to restrain the further flow of plastic. In the xe2x80x9cshort shotxe2x80x9d process, or when nearly filling the cavity in a xe2x80x9cfull shotxe2x80x9d process, there may remain unsightly visible hesitation lines or marks on the molding surface at the position of the first plastic injection.
Another method is disclosed in Japanese Patent Application No. 50-74660, where shut-off valves in runners connect the product cavity and xe2x80x9csecondary cavities.xe2x80x9d In this application, the mold cavity is filled with thermoplastic resin and then a core resin or gas is injected into the cavity while the thermoplastic resin in the mold is expelled from the mold cavity. After the thermoplastic resin fills the mold cavity, the core resin or gas is injected while the resin in the mold cavity is expelled.
It is an object of the present invention to provide an improved gas assisted molding process for use in the injection molding of plastic materials. It is another object of the present invention to provide an improved gas assisted plastic injection molding process which accurately controls the volume of plastic into the mold cavity and secondary cavity.
It is still another object of the present invention to provide a gas assisted plastic injection molding process in which a good reproducibility of the mold cavity surface on the molded product is produced and flow or weld marks and hesitation marks are minimized or eliminated. It is a still further object of the present invention to provide a gas assisted plastic injection molding process which has improved gas core out with larger expulsion of plastic, thereby reducing the weight and molding cycle time and securing more consistent wall or skin thickness.
It is still another object of the present invention to provide a gas assisted injection molding process which is suitable for use with a wide range of plastic materials, including gas-filled nylon and other filled materials and is also suitable for multi-product cavity molds.
The present invention provides a gas-assisted plastic injection molding process which meets the above objectives and provides an improved process for molding plastic products. In accordance with the present invention, molten plastic material is first injected into the mold cavity. One or more secondary cavities are positioned adjacent the mold cavity and each is provided with a certain volume. It is generally necessary to adjust the volume of the cavities after the first or subsequent mold test trial in order to match the volume of each secondary cavity with the volume of plastic required to be expelled from the molding cavity. This may be done by metal removal from the mold to increase the volume, or addition of metal to reduce the volume. Shutoff valves are positioned between the mold cavity and secondary cavities. The plastic material in the mold cavity is pressurized by the molding machine immediately after filling. The xe2x80x9cpacking pressurexe2x80x9d from the molding machine is maintained for a predetermined time period.
Thereafter, gas is injected into the molten plastic to continue pressuring the plastic material in the mold cavity. Again, after a predetermined time period to allow pressurization of the gas to press the plastic material against the surfaces of the mold cavity, the valves positioned between the mold cavity and the secondary cavities are opened enabling the expulsion of a volume of plastic sufficient to fill the mold cavity or cavities. Initially, the volume of plastic expelled is determined by the time at which the valves are opened. If the valve opening time is prolonged, more of the plastic solidifies in the mold cavity, i.e. the solid skin of the plastic will thicken, and less is expelled; conversely if the valve opening time is advanced, i.e. the delay in opening is reduced, the skin thickness is reduced and more plastic is expelled. When the balance of secondary cavity volume and valve opening time is optimised, the process can be operated consistently shot-after-shot in production. This precisely controls the volume of expelled plastic material. The gas pressure is again held in the mold cavity until the plastic material cools and solidifies. Thereafter, the gas is vented or exhausted from the mold cavity, the mold is opened, and the part is removed or ejected.
The volume of plastic material which is expelled from the mold cavity into the secondary cavity is not dependent on the timing of the gas injection. Instead, the volume of plastic expelled is dependent on the volume of the secondary cavity or cavities.
In an alternate embodiment, at least two secondary cavities are provided. Each are connected to the mold cavity with a separate valve. Each of the secondary cavities is connected to the article mold cavity by a flow runner or conduit in which there is a valve member. The operation of each of the valves is independently controlled and timed. The opening of the valves can be sequentially timed to optimize the required expulsion of plastic from the mold cavity and from different positions in the mold cavity.
In another embodiment of the invention, the plastic expelled by the gas is forced into the injection machine barrel either separately or in combination with one or more secondary cavities.
In a still further embodiment, the opening and closing of the valve members may be operated to allow the flow of plastic from the mold cavity to the secondary or overflow cavities by the application of a selected pressure exerted by the injection of gas. The injection of the gas would transmit pressure to the plastic material, which in turn will open the valve members and overcome a preset closing force. A preset closing force may be applied to the valve member by mechanical springs, pneumatic mechanisms, hydraulic mechanisms, electrical mechanism, or the like.
The opening of the valve members may be controlled in any conventional manner, such as mechanical, pneumatic, hydraulic, electric, or other means. The control can be either digital or computer timed and can be external or integral with the gas pressure control means.
Other objects, features, and benefits of the present invention will become apparent from the following specification when viewed together with the accompanying drawings and appended claims.